Microsoft-Graph: Our application can hardly ever receive the chat message subscription's callback

Question

Because of chat message's subscription only supports to bate version,so we choose the bate. We subscribe the chat message , response 200 status,and we can find subscription info in graph API '/beta/subscriptions' . but when we send the message on teams,we can hardly ever receive the callback in notify API.We checked the response、nginx、firewall,no any receive records. We used this API about 1.5 years,and this situation began in May this year. Thanks!